1. Field of the Invention
This invention relates to a surfactant composition of gemini surfactants and one or more co-amphiphile(s), to a process for preparing such composition, and to its uses, especially as emulsifiers and dispersants.
2. Description of the Prior Art
Mixtures of surfactants and co-amphiphiles and their use in emulsions or dispersions are well known. These mixtures have been designed for optimising the surfactant molecules pattern at the phase boundaries. When making emulsions, it is often intended to form liquid-crystalline, lamellar phases, which can be utilised to stabilise the emulsion.
Proportionate mixtures of special surfactants and co-amphiphiles are employed according to their applications. For those applications which require a high electrolyte content in aqueous solutions or emulsions/dispersions, ionic surfactants have up to now been considered to be largely unsuitable. In order to improve the selection of appropriate surfactants, efforts have been made to quantify or classify their polarities, thus enabling a more reliable assessment of the properties and application patterns. The HLB concept and its variations is mentioned here as an example of a simple approach.
Gemini surfactants, both as nonionic and cationic variants, have been known for more than twenty years, but it was not earlier than in the '90s that they stirred up a deeper interest. For a survey of the state of the art, see R. Zana, “Dimeric (Gemini) Surfactants” in Novel Surfactants Preparation, Applications and Biodegradability, C. Holmberg (ed.), Marcel Dekker (1998), p. 241.
Most of the recent publications deal with ionic gemini surfactants because these materials significantly enhance the interfacial activity, for instance expressed in terms of the critical micelle formation constant or the reduction in surface tension of water, in comparison with conventional surfactants.
At first gemini surfactants were mainly recommended for use in detergents and cleaning products.
JP-A 08/268 865 also describes the use of gemini surfactants in cosmetic preparations. In the formulations disclosed therein conventional anionic surfactants have been exchanged for anionic gemini surfactants, which are considerably less irritant to the skin, without laborious modification of the formulations.
EP-A-0 697 244 discloses amphoteric gemini surfactants, which can also be mixed with other anionic, nonionic, cationic, or amphoteric surfactants. Said surfactants are reported to be useful in detergents. The gemini surfactants (gemini amides) described in WO 95/19953 can be employed among others as components in customary cleaning preparations. In WO 95/19955 gemini polyethers have been disclosed as another class of gemini surfactants, which are useful for the same application mentioned hereinbefore. Mixtures of alkoxylated bisalkylphenol gemini surfactants and other surfactants are known from WO 97/23449.
In JP-A 11/60430 and JP-A 11/60437 the use of anionic gemini surfactants in cosmetics has been described. Reportedly, these surfactants can also be combined with other surfactants.
However, in contrast to what might be expected due to the substantial improvement in interfacial activity, no significant increase in efficiency or distinct improvement of application characteristics has been brought about by gemini surfactants as direct substitutes for conventional surfactants. The meagre advantage of such surfactant substitution cannot justify the effort of incorporating an additional building element in the basic structure.